Answer: More common in natural systems altered by human actions
Step-by-step explanation: Positive feedback loop is the loop that enhances the changes or improves the output. The positive feedback system usually faces instability because it moves away from the state of equilibrium.
The human action causes the system to get invoked with the change.E.g.- ripening of any fruit.
Other options are incorrect because the stability in substance's concentration occurs in negative feedback loop. Both the phenomena negative and positive feedback are common to occur in nature.